Twelve pairs of nerves—the cranial nerves—lead directly from the brain to various parts of the head, neck, and trunk. Some of the cranial nerves are involved in the special senses (such as seeing, hearing, and taste), and others control muscles in the face or regulate glands. The nerves are named and numbered (according to their location, from the front of the brain to the back).

Viewing the Cranial Nerves

Twelve pairs of cranial nerves emerge from the underside of the brain, pass through openings in the skull, and lead to parts of the head, neck, and trunk. The nerves are named and numbered, based on their location, from the front of the brain to the back. Thus, the olfactory nerve is the 1st cranial nerve, and the hypoglossal nerve is the 12th cranial nerve.

A cranial nerve disorder may result when the following are damaged or malfunction:

Some cranial nerve disorders interfere with eye movement. Eye movement is controlled by 3 pairs of muscles. These muscles move the eye up and down, right and left, and diagonally. The muscles are controlled by the following cranial nerves:

If one of one of these nerves is damaged, the muscles it controls may become paralyzed to varying degrees (called a palsy), and people may not be able to move their eyes normally. How eye movement is affected depends on which nerve is affected. People with one of these palsies may have double vision when they look in certain directions.

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Symptoms

Symptoms of cranial nerve disorders depend on which nerves are damaged and how they were damaged. Cranial nerve disorders can affect smell, taste, vision, sensation in the face, facial expression, hearing, balance, speech, swallowing, and muscles of the neck. For example, vision may be affected in various ways:

If one of the 2nd cranial nerves (optic nerve) is damaged, the affected eye is blind.

If any of the three cranial nerves that control eye movement (3rd, 4th, or 6th cranial nerve) is damaged, people cannot move their eyes normally. Symptoms include double vision when looking in certain directions.

If the 3rd cranial nerve (oculomotor nerve) is paralyzed, the upper eyelid is paralyzed. It droops down over the eye and interferes with vision.

If the 8th cranial nerve (auditory or vestibulocochlear nerve) is damaged or malfunctions, people may have problems hearing and/or vertigo—a feeling that they, their environment, or both are spinning.

Cranial nerve disorders can also cause various kinds of facial or head pain.

Diagnosis

Symptoms

Tests of cranial nerve function

Usually magnetic resonance imaging

When doctors suspect a cranial nerve disorder, they ask the person detailed questions about the symptoms and test the function of the cranial nerve by asking the person to do simple tasks, such as to follow a moving target with the eyes.

Imaging of the brain with magnetic resonance imaging (MRI) is often needed.

Testing Cranial Nerves

Cranial Nerve Number

Name

Function

Test

1st

Olfactory

Smell

The ability to smell is tested by asking the person to identify items with very specific odors (such as soap, coffee, and cloves) placed under the nose. Each nostril is tested separately.

2nd

Optic

Vision

The ability to see is tested by asking the person to read an eye chart. Peripheral (side) vision is tested by asking the person to look straight ahead while the doctor gradually moves a finger toward the person's center of vision from above, below, left, and right. The person is then asked to say when the finger is first seen.

Detection of light

The ability to detect light is tested by shining a bright light (as from a flashlight) into each pupil in a darkened room.

3rd

Oculomotor

Eye movement upward, downward, and inward

The ability to move each eye up, down, and inward is tested by asking the person to follow a target moved by the examiner.

Narrowing (constriction) or widening (dilation) of the pupil in response to changes in light

The pupils’ response to light is checked by shining a bright light (as from a flashlight) into each pupil in a darkened room.

Raises the eyelids

The upper eyelid is checked for drooping (ptosis).

4th

Trochlear

Eye movement downward and inward

The ability to move each eye down and inward is tested by asking the person to follow a target moved by the examiner.

5th

Trigeminal

Facial sensation

Sensation in areas of the face is tested using a pin and a wisp of cotton.

The blink reflex is tested by touching the cornea of the eye with a cotton wisp.

Chewing

Strength and movement of muscles that control the jaw are tested by asking the person to clench the teeth and open the jaw against resistance.

6th

Abducens

Eye movement outward

The ability to move each eye outward beyond the midline is tested by asking the person to look to the side.

7th

Facial

Facial expression, taste in the front two thirds of the tongue, production of saliva and tears, and control of a muscle involved in hearing

The ability to move the face is tested by asking the person to smile, to open the mouth and show the teeth, and to close the eyes tightly.

Taste is tested using substances that are sweet (sugar), sour (lemon juice), salty (salt), and bitter (aspirin, quinine, or aloes).

8th

Auditory (vestibulocochlear)

Hearing

Hearing is tested with a tuning fork or with headphones that play tones of different frequencies (pitches) and loudness (audiometry).

Balance

Balance is tested by asking the person to walk a straight line.

9th

Glossopharyngeal

Swallowing, the gag reflex, and speech

Because both the 9th and 10th cranial nerves control swallowing and the gag reflex, they are tested together.

The person is asked to swallow. The person is asked to say “ah-h-h” to check movement of the palate (roof of the mouth) and uvula (the small, soft projection that hangs down at the back of throat).

The back of the throat may be touched with a tongue blade, which evokes the gag reflex in most people.

The person is asked to speak to determine whether the voice sounds nasal (another test of palate movement).

10th

Vagus

Swallowing, the gag reflex, and speech

Control of muscle in some internal organs and the heart rate

This function is not tested as part of the cranial nerve examination.

11th

Accessory

Neck turning and shoulder shrugging

The person is asked to turn the head and to shrug the shoulders against resistance provided by the examiner.

12th

Hypoglossal

Tongue movement

The person is asked to stick out the tongue, which is observed for deviation to one side or the other.

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The peripheral nervous system consists of more than 100 billion nerve cells that run throughout the body like strings, making connections with the brain, other parts of the body, and often with each other. The peripheral nervous system has two parts: the somatic nervous system and the autonomic nervous system. The somatic nervous system is associated with which of the following body functions?

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